An energy storage device, especially a lithium secondary battery, has been widely used recently for a small-sized electronic device, such as a mobile telephone and a notebook personal computer, an electric vehicle, and an electric power storage. The electronic devices and vehicles may be used in a broad temperature range, for example, at midsummer high temperatures and frigid low temperatures, and therefore the energy storage device is required to have well-balanced and improved electrochemical characteristics in a broad temperature range.
Especially for preventing global warming, it is imperative to reduce CO2 emissions, and among eco-friendly vehicles having mounted thereon an energy storage equipment containing an energy storage device, such as a lithium secondary battery and a capacitor, early popularization of a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV) and a battery electric vehicle (BEV) is being desired. Vehicles may travel long distance and therefore may be used in regions with a broad temperature range covering from extremely-hot tropical regions to frigid regions. In particular, therefore, the in-vehicle energy storage device is demanded to have electrochemical characteristics that are not deteriorated even used in a broad temperature range covering from high temperatures to low temperatures.
In this specification, the term, lithium secondary battery, referred herein is used as a concept including a so-called lithium ion secondary battery.
A lithium secondary battery is mainly constituted by a positive electrode and a negative electrode, which contains a material capable of absorbing and releasing lithium, and a nonaqueous electrolytic solution containing a lithium salt and a nonaqueous solvent. Examples of the nonaqueous solvent used include a carbonate, such as ethylene carbonate (EC) and propylene carbonate (PC).
Known examples of the negative electrode include metal lithium, and a metal compound (a metal elemental substance, an oxide and an alloy with lithium, etc.) and a carbon material capable of absorbing and releasing lithium. In particular, a lithium secondary battery using a carbon material capable of absorbing and releasing lithium, such as coke, artificial graphite and natural graphite, has been widely put into practical use.
For example, it is known that, in a lithium secondary battery using a highly-crystalline carbon material, such as natural graphite and artificial graphite as the negative electrode material therein, a decomposed product and gas generated through reductive decomposition of the solvent in the nonaqueous electrolytic solution on the surface of the negative electrode during charging inhibit the electrochemical reaction favorable for the battery, which may worsen the cycle property of the battery. Deposition of the decomposed product of the nonaqueous solvent inhibits smooth absorption and release of lithium by the negative electrode, which may worsen the electrochemical characteristics of the battery on using in a broad temperature range.
Furthermore, it is known that a lithium secondary battery using a lithium metal or an alloy thereof, or a metal elemental substance, such as tin and silicon, or a metal oxide thereof as the negative electrode material may have a high initial battery capacity, but the battery capacity and the battery performance thereof, such as the cycle property, may be largely worsened since the micronized powdering of the material may be promoted during cycles, which brings about accelerated reductive decomposition of the nonaqueous solvent, as compared with the negative electrode formed of a carbon material. In addition, the micronized powdering of the negative electrode material and the accumulation of the decomposed products of the nonaqueous solvent would interfere with smooth absorption and release of lithium by the negative electrode, thereby often worsening the electrochemical characteristics of the battery on using in a broad temperature range.
On the other hand, it is also known that, in a lithium secondary battery using, for example, LiCoO2, LiMn2O4, LiNiO2, LiFePO4 or the like as the positive electrode, the nonaqueous solvent in the nonaqueous electrolytic solution locally undergoes partial oxidative decomposition at the interface between the positive electrode material and the nonaqueous electrolytic solution in a charged state, the decomposed product and the gas generated thereby may inhibit the electrochemical reaction favorable for the battery, and thereby the electrochemical characteristics of the battery may be worsened on using in a broad temperature range.
As described above, the decomposed product and the gas generated through decomposition of the nonaqueous electrolytic solution on the positive electrode or the negative electrode may inhibit migration of lithium ions or may swell the battery, which may worsen the battery performance. Irrespective of the situation, the multifunctionality of electronic appliances equipped with lithium secondary batteries therein is more and more enhanced and power consumption tends to increase. The capacity of lithium secondary battery is thus being much increased, and the space volume for the nonaqueous electrolytic solution in the battery is decreased by increasing the density of the electrode, by reducing the useless space volume in the battery, or the like. Accordingly, the current situation is that the electrochemical characteristics of the battery on using in a broad temperature range may be worsened even with decomposition of only a small amount of the nonaqueous electrolytic solution.
In order to increase safety, PTL 1 proposes a nonaqueous electrolyte secondary battery containing a molten salt, such as lithium acetate trifluoroborate, lithium methanesulfonate trifluoroborate, etc.; and it is suggested that these compounds are liquid at ordinary temperature (in the neighborhood of 25° C.) and do not substantially use an organic solvent, so that they are not substantially volatilized, and thus, the safety becomes high.
PTL 2 discloses a nonaqueous electrolytic solution which is a nonaqueous electrolytic solution to be used for a secondary battery provided with a negative electrode active material having Si, Sn, or Pb and is capable of improving battery characteristics by containing two kinds of first and second lithium salts.